The recent development of small and compact electronic parts such as semiconductor elements has created a need for electrical insulating materials with particular properties. Some electronic parts such as power transistors and thyristors, in particular, are liable to be damaged by or to deteriorate from the heat generated during operation. Accordingly, these electronic parts are usually protected by heat radiating, finned metal plates which are attached to the electronic parts through an insulating sheet of high thermal conductivity.
Conventionally, mica sheets and polyester films have been used to make these insulating sheets. Grease is applied to these insulators to ensure close adhesion of the insulators to the electronic parts. Even though these materials are inexpensive and therefore desirable in terms of cost, their thermal conductivity is insufficient. Furthermore, the application of grease to the insulating material is troublesome and likely to flow out or deteriorate due to heat. The use of those conventional sheets or films has, therefore, not been satisfactory.
Insulating materials made of a synthetic rubber containing heat conductive powder, such as alumina or zinc white powder, as a filler are also known. With such materials, the heat conductivity can be enhanced by increasing the volume ratio of the heat conductive inorganic powder to the insulator. Unfortunately, increasing the volume ratio of the filler usually decreases moldability and mechanical strength of the insulator. In addition, the insulating material becomes brittle and cracks when bent when the amount of filler in the insulator is increased. As a result, in practical applications using a synthetic rubber insulator containing an inorganic powder, the volume ratio of the powder to insulator cannot be increased enough to yield a satisfactory thermal conductivity.
A synthetic rubber insulator incorporating a woven or nonwoven fabric or staple fibers has also been proposed. The incorporation of a woven or non-woven fabric or staple fibers into the insulating sheet improves the strength of the sheet. However, because the volume ratio of filler to insulator cannot be increased, the thermal conductivity of the sheet is not necessarily satisfactory.
It is, therefore, an object of the present invention to provide an electrically insulating material with high thermal conductivity which can be placed between electronic parts and a heat radiator.
It is a further object of the present invention to provide an insulating material which is flexible, strong, and can be posititioned around electrical parts without difficulty.
It is still further object of the present invention to provide an insulating material which can contain a high percentage by volume of heat conductive inorganic powder to bonding agent.
Another object of the present invention is to provide an insulating material which can be further strengthened by reinforcement with woven or non-woven fabric or staple fibers.
Based on the fact that polytetrafluoroethylene resin, hereinafter referred to as "PTFE", can easily be fibrillated by an applied compressive shearing force, the present invention comprising an insulating material made of heat conductive inorganic powder held together by a bonding agent containing fibrillated PTFE and synthetic rubber was developed wherein the volume ratio of inorganic powder filler to bonding agent can be increased to an extremely high value.